Generally, IPM motors, which use expensive permanent magnets, have high cost but higher efficiency than induction motors. Therefore IPM motors are widely used for driving motors and power generating motors for hybrid vehicles and electric vehicles, and motors for home electric appliances, various machine tools and industrial machines.
An iron core of IPM motors is constituted by a stator and a rotor. Since an AC magnetic field is directly applied to the iron core on the stator side via windings, the iron core on the stator side must have high magnetic permeability and high volume resistivity so as to reduce iron loss. Hence electromagnetic steel sheets, of which the soft magnetic characteristics have been improved by adding Si to ultra low carbon steel, are used for the iron core on the stator side.
The iron core on the rotor side, on the other hand, mainly plays a role of increasing magnetic flux density as a yoke, since a permanent magnet is embedded in the iron core on the rotor side. The iron core on the rotor side is subject to minor influence of the AC magnetic field generated from the stator side, but this influence is limited. Therefore in terms of characteristics, it is not necessary to use electromagnetic steel sheets, which are advantageous for the iron loss characteristic, for the iron core on the rotor side. However, the same electromagnetic steel sheets as the stator side are also used for the iron core on the rotor side because the product yield of the electromagnetic steel sheets drop and manufacturing costs of the motor increase if the electromagnetic steel sheets are used only for the stator.
In the case of mounting an IPM motor on a vehicle, miniaturization is sought for the IPM motor because a vehicle must be compact and light. In this case, the rotational speed of the rotor is increased in order to obtain a motor output (torque) equivalent to or more than a conventional motor despite the miniaturization. Generally the efficiency of a motor improves as the rotational speed of the rotor increases. However in the case of an IPM motor, an induced electromotive force is generated on the stator windings by the rotation of the embedded permanent magnet. The induced electromotive force increases as the rotation speed increases. Then the motor can no longer rotate when the electromotive force exceeds the input voltage.
Therefore in an IPM motor, a field-weakening control, which suppresses the induced electromotive force by generating a magnetic flux from the stator side in a direction to cancel the magnetic flux of the permanent magnet, is performed when the motor is operated in a high-speed rotation range, as disclosed in Japanese Patent Application Laid-Open No. 2000-278900 A, for example. Although operation in a high-speed rotation range becomes possible, the field-weakening control decreases the motor torque because power is used for cancelling the magnetic flux of the permanent magnet. According to Japanese Patent Application Laid-Open No. 2000-278900 A, electric energy to be used for the field-weakening control can be decreased by improving the shape of the magnet.
On the other hand, even if the IPM motor is miniaturized, there is a problem where the centrifugal force that acts upon the permanent magnet embedded in the rotor increases to damage the rotor if the rotational speed of the rotor is increased so as to obtain a torque equivalent to or higher than conventional motors. To prevent damage, it is preferable to use a material having high yield strength for the material of the rotor. For example, in the case of non-orientated electromagnetic steel sheets (35A300) containing about 3% Si, the yield strength after magnetic annealing is approximately 400 N/mm2. Therefore in the case of a relatively large IPM motor where the diameter of the rotor is 80 mm or more, the limit of the rotational speed at which damage is not caused is about 20,000 rpm, although the value is somewhat different depending on the structure of the rotor. Various studies have been made to increase the yield strength of the iron core based on the electromagnetic steel sheets, but still the yield strength is at most about 780 N/mm2.
In this way, an attempt is made to obtain higher torque by increasing rotational speed using the conventional rotor iron core made of electromagnetic steel sheets when an IPM motor is miniaturized, there is a limit on increasing the rotational speed because there are problems where torque decreases in the high-speed rotation range even if field-weakening control is performed, and the rotor could be damaged by the centrifugal force that acts on the permanent magnet.
As a method for suppressing damage to the rotor iron core due to high-speed rotation, Japanese Patent Application Laid-Open No. 2009-153230 A, for example, proposes to increase strength by using a material with softness and hardenability for the material of the rotor iron core, and selectively quenching only a bridge portion near a permanent magnet insertion hole and the vicinity thereof. Furthermore, Japanese Patent Application Laid-Open No. 2009-046738 A, for example, proposes to use not electromagnetic steel sheets but a material with high strength and a high saturation magnetic flux density for the material of the rotor iron core.
While developing steel sheets for a rotor for high-speed rotation, the present inventors manufactured experimental IPM motors using various steel sheets as materials, evaluated the performance of the motors, and as a result discovered that a large output torque can be obtained in a high-speed rotation range where the field-weakening control is performed by adjusting the coercivity of the base steel sheets. By obtaining a larger output torque, the rotor can be rotated at a higher rotational speed.
In Japanese Patent Application Laid-Open No. 2000-278900 A, an attempt was made to decrease the electric energy used for the field-weakening control by improving the shape of the magnet, but adjusting the coercivity of the base steel sheets was not considered here. In Japanese Patent Application Laid-Open No. 2009-153230 A and Japanese Patent Application Laid-Open No. 2009-046738 A as well, adjusting the coercivity of the base steel sheets is not considered. In other words, in conventional configurations, adjusting the coercivity of the base steel sheets is not considered, hence the output torque in a high rotation range becomes small, and the maximum rotational speed accordingly becomes low.